Many industrial plants and municipal facilities generate waste products that are partial emulsions of water, oil and solids and are referred to as "sludges." For example, petroleum refineries and manufacturing plants generate significant quantities of oily waste products that contain a variety of solids, such as suspended carbonaceous matter and inorganic matter such as rust scales, catalysts fines and the like. Furthermore, most of this waste is classified as hazardous and, therefore, must be treated before disposal in regulated landfills to meet concentration limits for certain organic compounds, cyanides and several heavy metals.
Refinery waste sludges are among the most difficult emulsions to break. Yet the disposal criteria for solids are expensive to reach, necessitating the separation of the water, oil and solid components to minimize the amount of wastes that require disposal. Further, the hydrocarbons can be recycled in the plant's processes and the water can be treated per normal waste water treatment processes, if sufficient amounts of solids are removed. Therefore, although refinery waste sludges are very difficult to break, it is often the most economical means of treating the waste.
An emulsion may be present in oily waste products other than refinery waste sludges. Emulsions may also be present in used oil. In the oil exploration industry, for example, drilling fluid may become saturated with cuttings, oil, water, and other materials. The saturated drilling fluid is no longer usable and must be discarded. The saturated drilling fluid may include an emulsion that includes the oil constituent of the drilling fluid. Unless the oil constituent of the saturated drilling fluid can be separated from the emulsion, the oil constituent will be discarded with the remainder of the saturated drilling fluid. If the saturated drilling fluid could be demulsified, the oil of the drilling fluid could be recovered and recycled as drilling fluid. Similarly, an emulsion may be present in other forms of used or spent oil. In the case of motor oil, the oil may become soiled with dirt and metal particles during use and an emulsion may be formed of the oil and other constituents of the used oil. Unless the oil constituent of the motor oil can be separated from the spent motor oil, the oil constituent will be discarded with remainder of the spent motor oil.
In order to understand the technology used to separate waste sludges, it is useful to review the theory of emulsion. An emulsion is an intimate mixture of two immiscible liquids, such as oil and water. Two types of oil and water emulsions are commonly encountered, based on the relative amounts of oil and water. The first is an oil dispersed in water (oil in water) emulsion, and the second is a water dispersed in oil (water in oil) emulsion.
A stable oil in water emulsion consists of electrically charged oil droplets dispersed in a polar medium such as water. The violent mixing and shearing of oily wastewater in transfer pumps disperses the minute oil droplets throughout the water, and the friction between the oil and water phases creates static electrical charges at the oil and water interphase and helps to stabilize the emulsion.
This emulsion can be further stabilized by a variety of chemical and physical mechanisms. Surfactants (such as soaps, cresylates, sulfides, and electrolytes) usually carry an electric charge and travel to the oil/water interface (the interfacial film) of the droplets, thus reinforcing the repulsion of the droplets. Fine, solid particles may also stabilize an emulsion if the particles are of the correct size and abundance. The solid particles adsorb at the oil/water interface, reinforce the interfacial film and prevent the droplets from coalescing. Thus, solid particles also reinforce the stability of the dispersion.
The "breaking," or "resolution," of an oil in water emulsion is done by neutralizing the charges at the surface of the droplets. This is performed with a cationic emulsion breaker because the dielectric constants of oil and water cause the oil droplets to carry a negative charge in water. Lowering the pH, with sulfuric acid for example, can also help by converting any carboxyl ions present in the surfactant into carboxylic acid. Once the charges are neutralized, the phases can be gravity separated in an API or CPI separator. This separation can be significantly accelerated by centrifuging the mixture. Treatments to break waste sludges include also floatation, ultrafiltration, activated carbon adsorption, coalescence and solvent extraction.
Water in oil emulsions are viscous, concentrated substances formed when oil comes into contact with water and solids. Where agitation is present, the water becomes dispersed in the oil. Finely divided solids, ranging from colloidal to 100 microns, are particularly effective in stabilizing these emulsions. Other stabilizing agents include soaps, sulfonated oils, asphaltic residue waxes, salt sulfides and mercaptans.
The breaking of a water in oil emulsion can be done with physical methods such as heating and centrifugation. Chemical treatment of water in oil emulsions is directed at destabilizing the dispersed water droplets and solids or at destroying the emulsifying reagents. Usually anionic reagents are employed to destabilize the water droplets because the water droplets tend to be positively charged. Acidification may also be effective if the acid dissolves some of the solids and thus reduces the amount of stabilizing solids. Another method of treatment involves potent demulsifying agents carrying both hydrophilic and lipophilic groups. The demulsifying agent displaces the original emulsifying agent because it has more potent surface active agents. Heating reduces the viscosity of the emulsion and increases the solubility and diffusion of the demulsifying agent in the emulsion. Usually, thorough mixing and heat are both necessary to help disperse the demulsifying agents in the emulsion and to facilitate the separation of the phases once the charges are neutralized. Here again, centrifugation is the key to accelerating the separation.
The emulsions encountered in refinery waste are very diverse because the waste itself is a very heterogeneous material coming from many different processes. Therefore, the separation of refinery waste into its constituents is a very tricky operation, far removed from the cleanliness of the theoretical resolutions described above. The state of the art includes multiple technologies, all attempting to enhance the efficiency of charge neutralization and the settling of the demulsified residual mixture. Myriads of chemical formulations have been created for breaking emulsions, showing by their number and diversity that there is no magic potion that can universally break refinery waste sludges.
Physical methods of resolving refinery waste sludges include sophisticated centrifuges that have been designed to apply high G forces and long residence times (a high .SIGMA. factor) (see e.g., U.S. Pat. Nos. 4,810,393; 5,443,717). In U.S. Pat. No. 5,443,717 (the '717 patent), for example, a process for producing a quench stream for use in the quench cycle of a delayed coking process is described. A waste stream containing water, organic compounds and solids is treated by centrifugation in a vertical disk centrifuge. After being separated from the oil and water components of the stream, the particulate matter leaves the centrifuge at a very high exit speed and the impact of the solids on the exit shield causes attrition of the particle size. This particle attrition makes the solids produced thereby particularly suitable for use in a coker quench stream.
However, this method requires the use of an expensive centrifuge with a high .SIGMA. factor and is thus economically disadvantaged, not only because of the high initial outlay for such sophisticated equipment, but also because of its maintenance costs. Further, because the inventors employed particle attrition means only after the separation of the three phases of the waste sludge, they failed to take advantage of shear forces to assist in the resolution of the emulsion. Even if the inventor had been aware of the possibility of resolving emulsions with shear force, the design of the high speed disk centrifuge would not allow the application of shear forces to the emulsion because most of the attrition of the solids due to shear is traced to the projection of solids on a shield as they exit the centrifuge bowl.
U.S. Pat. No. 4,810,393 relates to a process for the resolution of oily sludges that consists of heating the suspension at a temperature exceeding 60.degree. C., and separating the heated suspension in the centrifuge described in the '717 patent into an aqueous phase, an oily phase and into sediments. A flocculent is added to the sediments and, while the temperature is at least 5020 C., the water is squeezed out by pressing on a small mesh filter. The sediments are then eliminated either by discharging or by incineration. The heating assists in reducing the viscosity of the oil and increases the solubility and diffusion of the flocculating agent in the emulsion. This process causes particle attrition and therefore the solids can be disposed of in a coker quench stream. The method, however, employs the centrifuge described above, thus contributing to the expense of the method.
Another approach for resolving tight emulsions has been to apply pressure and heat in a pressure cooker. For example, U.S. Pat. No. 4,938,876 describes a process for resolving emulsions by heating the mixture to at least about 115.degree. C., rapidly cooling the mixture to below 100.degree. C., and separating the resulting phases. Preferably, the invention includes the step of adding a flocculent prior to cooling the mixture. The expansion method of achieving rapid cooling apparently serves to break the emulsion by rupturing the microstructure of solids protecting the oil/water interface. Once the solids settle out of the emulsion, they can be separated by centrifugation, settling or filtration. This method is disadvantaged in that applying pressure to bring water to 115.degree. C. and expanding to flash cool necessitates the use of expensive pressure vessels. Further, the efficacy of the process has not been demonstrated on a commercial scale.
Some of these methods work reasonably well to break emulsified material into its constituents and separate them. Usually, however, the solids are not completely de-oiled, making them unsuitable for recycling as a coke quench. Therefore, the solids (if hazardous) must be disposed of either as an alternate fuel for burners or industrial furnaces, or further thermally de-oiled and sent to landfills. Neither solution is as economical as recycling the solids in a coker. Therefore, an economical, efficient means of completely resolving a wide variety of very tight waste emulsions, including refinery waste sludges, is needed.